Re: 99 Taurus SHO Overdrive issue
If the transmission light is on, codes will be stored for up to 50 restarts.
I believe there is an OD solenoid within the transmission that might be faulty. It might also be a couple other things. A bad IAC (idle air controller) can cause havoc on the transmission, along with a throttle position sensor. A bad TPS is often misdiagnosed as a major transmission problem. The IAC can be cleaned. The TPS cannot.
The other problem might be with the A/F ratio. The MAF sensor will get dirty eventually and start throwing the PCM incorrect readings that can result in anything from engine misfire to rpm fluctuations as you mentioned. In most cases the MAF can be cleaned with yes, you guessed it - MAF cleaner, thus saving yourself a couple hundred dollars.
Of course, worst case is that there are multiple problems, or the one problem led to another as more than one sensor has failed. The utmost worst outside of a transmission, is a bad PCM or wiring harness issue that will put the cost in the hundreds of dollars.
Most transmission sensors require dropping the pan, at which time a transmission service is required (about $100 or so). In some cases, a service isn't necessary but if it's been 40K miles since the last service, you're due.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!